Generally, an air conditioner is provided in a factory within which a machine tool is installed. Once the air conditioner is activated, the room temperature in the factory changes to gradually settle to a certain temperature. In particular, when the room temperature changes, temperature differences are generated between the high places and the lower places in the factory. Further, differences in the temperature occur between areas near the outlet of the air conditioner, the doors and/or the windows of the factory and the areas distant therefrom. The magnitudes of the temperature differences and the temperature distributions change with the passage of time. Such changes in the temperature and the temperature distribution where a machine tool is installed result in temperature differences between each part of the machine tool, such as the front, back, upper and lower portions of a column or a workpiece mount. Thermal deformation, such as inclination or warping is generated by the uneven temperature of each part of he of machine tool. If a workpiece is machined under such a condition, the machining accuracy is decreased by the thermal deformation.
Japanese Examined Patent Publication No. 61-26453, as a first prior art for preventing thermal deformation of a column, discloses an electrical machining apparatus in which a pressurized liquid is filled within cavities of a column and a bed to enhance the thermal rigidity by increasing the thermal capacity of the column and the bed.
Japanese Unexamined Patent Publication No. 10-249660, as a second prior art, discloses a machine tool having a column with a cooling feature. The machine tool includes a cooling unit and coolant tank which are disposed within cavities, which have been made at the four corner portions of the column during the casting, for circulating the coolant.
In the electrical machining apparatus according to the first prior art, temperature differences are generated between each part of the column due to the changes in room temperature accompanied with the passage of time from the early morning through the noon to the night and the temperature distribution in the factory because the column and the bed are simply filled with a liquid. Therefore, the temperatures of the respective front, rear, top and bottom portions of the column cannot be the same, which results in a thermal deformation to reduce the machining accuracy.
In the machine tool according to the second prior art, the coolant, which is cooled by coolant supply and recovery means, is recirculated into the column. Therefore, a complex and large apparatus is required. Further, the coolant is constantly circulated even when a work is not machined. Therefore, there is a problem that the energy consumption is large.